1. Field of the Invention
The invention relates to signal processing, and more particularly to coder-decoders (CODEC).
2. Description of the Related Art
Coder-decoders are circuits converting signals between analog and digital forms. For example, after analog signals are converted to digital signals by coder-decoders, the digital signals can then be processed by digital processors of computers or other devices to generate audio or video samples. Conventional audio and video samples can be further compressed to reduce the transmission bandwidth thereof.
Multiple access is a modulation technique used in satellite and long-distance communication. A communication satellite or switching center can simultaneously service multiple satellite terminals or mobile connections via a single communication connection link with multiple access modulation. Because bandwidth of mobile communication systems is limited, the system only assigns bandwidth to clients when clients initiate a communication session and revokes assigned bandwidth for use of other clients when the communication session is over. Thus, the mobile communication system must dynamically allocate bandwidth, and multiple access modulation is suitable for mobile communication systems to reduce bandwidth resource requested by a single client. Current multiple access modulation methods include a Time Division Multiple Access (TDMA) method, a Frequency Division Multiple Access (FDMA) method, and a Code Division Multiple Access (CDMA) method.
FDMA slices a frequency band into multiple channels of equal bandwidth, each FDMA channel carrying data of a specific client. TDMA slices a transmission period into multiple time slots of equal length, each transmitting data of a specific client. CDMA modulates data of multiple clients with spreading codes to obtain a CDMA signal, and a CDMA receiver retrieves data of a specific client from the CDMA signal with the specific spreading code corresponding to the specific client. Thus, the bandwidth of a mobile communication system can be dynamically allocated to multiple clients simultaneously accessing the system.
A conventional codec requires an analog-to-digital converter for conversion of audio or video signals. If a codec has multiple audio inputs or video inputs, the codec requires multiple analog-to-digital converters accordingly. The multiple analog-to-digital converters occupy considerable chip area of a codec chip, increasing production costs. For example, two analog-to-digital converters can substantially occupy 40% area of a codec chip, complicating chip design and increasing production cost. If a codec handles multiple analog inputs with a single analog-to-digital converter, cost of the codec is effectively decreased.